1. Field of the Invention
This invention relates generally to external structural enhancement of the wall structure of pressure containing metal vessels such as pipes, tanks, containers, etc. to minimize the potential for pressure induced failure thereof, to accommodate deterioration thereof by corrosion, erosion, surface divots, etc. and to enhance the pressure containing capability thereof. More specifically, the present invention concerns the provision of a novel method and apparatus for bonding at high temperature a chemical resistant wrapping of composite material about a pressure containing vessel, such as a pipeline, tank or the like, and curing it in place by application of predetermined heat for a predetermined period of time to thus provide an external structural component that enhances the structural integrity of the pressure containing vessel. Even more specifically, the present invention is directed to the application of a chemical resistant wrapping of biaxially or triaxially woven fiberglass and a polymer composite material in the form of a high temperature range epoxy, such as that sold under the trademark BTL Methylon resin, which becomes cross-linked in the presence of a phenylic resin and a catalyst, such as orthophosphoric acid, at a temperature in the range of from about 18020 F. to about 350.degree. F.
2. Description of the Prior Art
Cylindrical metal objects such as pipes, tanks and other pressure containing vessels are often exposed to hazardous environmental conditions, including corrosive conditions, erosive conditions and conditions having significant changes in temperature. Such cylindrical objects are often subjected to significant internal pressure, such as the pressure of fluid flowing through a pipeline, the pressure of fluid contained within a pressure vessel, the hydrostatic or vapor pressure of a liquid being contained, etc. When the external or internal wall surface of a pipe or other pressure containing vessel becomes structurally degraded by corrosion or erosion the wall thickness of the vessel becomes diminished in the corroded or eroded region thus diminishing the safe pressure containing capability of the vessel. Corrosion or erosion of the vessel wall thickness can result from the corrosive environment in which the vessel is located or the corrosive or erosive nature of the product flowing through or being contained by the vessel. Diminished vessel wall thickness can also be caused by mechanical damage to the vessel, such as by impact with other objects, scraping or gouging of the vessel wall by other objects, etc. This diminished wall thickness makes the vessel subject to pressure induced rupture at the region of diminished wall thickness. It is desirable to provide a structural repair system that has the inherent effect of restoring the thickness of the vessel wall from the standpoint of resistance to pressure responsive rupture. When pipelines become so degraded they will typically be re-rated to a lower pressure handling capability, so that productive service of the pipeline is diminished. It is desirable to provide a protecting and structurally enhancing external layer of material which is capable of being applied insitu and which can permit an otherwise degraded pipeline to be restored to it originally designed pressure containing capability.
It has been determined that rupture of corroded, eroded or otherwise damaged pipes, tanks and other pressure containing metal objects can be significantly retarded by the addition of an external reinforcement layer that intimately engages and preferably is bonded to the external surface thereof. It has also determined that the addition of an external reinforcement layer of corrosion and erosion resistant material to pressure containing vessels can significantly retard further deterioration of the wall thickness of the vessel and can thereby significantly extend the service life of pipes, tanks and other pressure containing vessels. It has also been determined that structural repair of this nature can be accomplished through application of polymer, fiberglass and other reinforcing compositions to the external surface of vessels without taking the vessels out of service during the period repairs are being accomplished.
It is desirable to provide pipes and other pressure containing vessels with an external layer of structural material that enhances the structural integrity thereof and which provides a durable external protective layer that provides pressure containing metal vessels with protection against corrosion and erosion and which also provides such vessels with an external insulation barrier to enhance the resistance of the vessels to thermal expansion and contraction.
Various non-metal layers of material have been applied to pipes and other pressure containing vessels for protection against stress fractures and to provide for enhanced pressure containing capability as well as correcting vessel wall defects caused by corrosion, erosion, mechanical gouging, etc. It is known to cement wrapped strips of pre-cured polymer composite wrapping material about pipes to correct wall defects, retard stress fracturing and to enhance the structural integrity and pressure containing capability of the pipe. Such materials, however, being strips of pre-cured polymer material, while effective for being wrapped about straight sections of pipe, cannot be used effectively for wrapping various types of pipe fittings, such as "T's", elbows, and the like. This undesirable factor significantly limits the commercial viability of this particular product. Additionally, the pre-cured state of this type of pipe wrapping material makes bonding of the wrappings or laminations of the material about the pipe and bonding of the overlapping layers of the material to each other virtually impossible to accomplish. Such pipe wrapping material is subject to de-lamination and is also subject to leakage as pressurized material such as liquid leaks from a wall defect opening in the wall structure of the pipe and migrates along the interface of the polymer wrapping material with the pipe surface, propagating de-lamination until a point of leakage, typically an end of the repair, is encountered. Moreover, the initial structural integrity of pipe repair or structural enhancement using pre-cured polymer composite pipe wrapping material is determined by how well the material is layered as it is wound about and drawn tightly about the pipe.
A serious limitation of most polymer wrappings of pipe and other pressure containing vessels for the purpose of wall repair or wall strengthening is that such materials often begin to degrade when the ambient temperature or the temperature of the pipe or vessel becomes elevated above a relatively low temperature, such as 180.degree. F., for example. Thus, where the ambient temperature of the pipe is above 180.degree. F. such a composite pipe repair can neither be properly installed or properly function. In the chemical and petrochemical industries and in certain temperate regions of the world such a low temperature repair system will not be found satisfactory. It is desirable therefore to provide a pipe wrapping system having the capability of being installed and having the capability of effective operation under conditions of elevated temperature, such as in the range of about 275.degree. F. to 375.degree. F. for example. It is also desirable to provide a multilayer wrapping of polymer material about pipe and other pressure containing vessels wherein the various layers thereof are integrated by polymerization and will not become de-laminated.
After a pipeline has been installed for a number of years the conditions near the pipeline right of way can change such that its safety to the immediate environment requires a re-rating change, typically lowering the maximum operating pressure rating of the pipeline. Also, in the event of deterioration of the pipeline by corrosion, the pressure rating of the pipeline may need to be lowered to maintain the same safety standards of its installation, a condition known as "de-rating". At times, it may become desirable to increase the pressure rating of a pipeline to enhance its flow handling production or to provide the pipeline with external reinforcement to maintain the pressure rating for which the pipeline was originally designed. Heretofore, application of an epoxy or other polymer externally of the pipe to enhance the pressure containing capability thereof has not been a viable option. It is desirable, therefore, to provide a novel polymer composite repair method which is designed to remediate corroded and mechanically damaged pipes or vessels and which may also be utilized to permit re-rating of the pipeline to a higher maximum operating pressure rating to thus enhance the productivity of the pipeline. It is also desirable to provide an external wrapping system which may also be employed to enable insitu re-rating of the vessels or pipes to a safe and higher pressure containing capability.
It has been determined through testing that pipes, especially when deep pressure cycled in cold conditions, can be subject to polymer disbonding from the external pipe surface. Also, if the polymer material is pre-cured and then strips of precured polymer are wrapped about the pipe and cemented or bonded in wrapped condition about the pipe, deep cycling of the pipe can also result in disbonding between the wrapped layers. Cathodic disbandment of pipe wraping can also be a problem when pipes are buried and subject to electrolysis. It is desirable therefore to provide a method and materials for application of polymer wrapping to pipes, tanks and other vessels and which is chemically resistant to mild acid and alkaline environments and which effectively meets the ASTM G8 standards regarding cathodic disbonding for pipes as well as ASTM G9 standards regarding water penetration into pipe coatings.
Particularly when pipe is subjected to seawater, such as is the case with offshore oil and gas production, it is desirable to have the capability for cleaning and protecting the pipes with polymer coatings to resist the effects of corrosion and marine life. For purposes of repair, it is desirable to provide a polymer composite repair composition having the capability for use insitu for repairing and coating pipes in the subsea environment. Currently, a commercially available polymer composition that may be used for protective wrapping of pipes, tanks and other structures in an underwater environment is not known.